SUSCEPTIBILITY AND IMMUNITY. 247 



serum, quite independently of the cellular elements contained in it 

 in the circulation, has decided germicidal power for certain patho- 

 genic bacteria, and that the blood serum of the rat and other animals 

 which have a natural immunity against anthrax is especially fatal 

 to the anthrax bacillus. 



Numerous experiments have been made during the past two or 

 three years with a view to determining whether pathogenic bacteria 

 are, in fact, destroyed within the leucocytes after being picked up, 

 and different experimenters have arrived at different conclusions. 

 In the case of mouse septicaemia, already alluded to, and in gonor- 

 rhoea, one would be disposed to decide, from the appearance and ar- 

 rangement of the pathogenic bacteria in the leucocytes, that they are 

 not destroyed, but that, on the other hand, they multiply in the in- 

 terior of these cells, which in the end succumb to this parasitic in- 

 vasion. In both of the diseases mentioned we find leucocytes so 

 completely filled with the pathogenic microorganisms that it is diffi- 

 cult to believe that they have all been picked up by a voracious pha- 

 gocyte, which has stuffed itself to repletion, while numerous other 

 leucocytes from the same source and in the same microscopic field of 

 view have failed to capture a single bacillus or micrococcus. More- 

 over, the staining of the parasitic invaders, and the characteristic ar- 

 rangement of the " gonococcus " in stained preparations of gonorrhceal 

 pus, indicate that their vitality has not been destroyed in the interior 

 of the leucocytes or pus cells, and we can scarcely doubt that the 

 large number found in certain cells is due to multiplication in situ 

 rather than to an unusual activity of these particular cells. But in 

 certain infectious diseases, and especially in anthrax, the bacilli in- 

 cluded within the leucocytes often give evidence of degenerative 

 changes, which would support the view that they are destroyed by 

 the leucocytes, unless these changes occurred before they were picked 

 up, as is maintained by Nuttall and others. We cannot consider 

 this question as definitely settled, but, in view of the importance 

 attached to the theory of phagocytosis by many pathologists and bac- 

 teriologists, we reproduce here a recent paper by Metschnikoff in 

 which his views are fully set forth : 



LECTURE ON PHAGOCYTOSIS AND IMMUNITY. 1 



It is not possible to study the bacteriology of disease without noticing 

 that, while in many cases the invading microorganisms are to be found 

 solely in the fluids of the. body, in not a few affections they present them- 

 selves in the interior of certain cells, and this either partially some being 

 within the cells, others free in the blood plasma and the lymph that bathes 

 the various tissues or exclusively, all the bacteria that are visible being 



1 Delivered at the Institut Pasteur, December 29th, 1890, by Dr. Elias Metschni- 

 koff, Chef rte Service de 1'Institut Pasteur, Paris ; late Professor of Zoology in the 

 University of Odessa. 



